As there is a constantly growing amount of mobile data traffic, spectrum resources become increasingly insufficient, and a demand for the amount of traffic may not have been satisfied by deploying a network and transmitting traffic only over spectrum resources in licensed frequency bands. In view of this, transmissions in a Long Term Evolution (LTE) system may be deployed over spectrum resources in an unlicensed frequency band (which is referred to as Unlicensed LTE, or simply U-LTE or LTE-U) to improve user experience and extend a coverage area.
Spectrum resources in an unlicensed frequency band have not been planned for any particular application system, but may be shared by various wireless communication systems (e.g., Bluetooth, Wireless-Fidelity (WiFi), etc.), where the various systems access the shared spectrum resources in the unlicensed frequency band by preempting the resources. Ongoing researches are focused on the coexistence of LTE-U schemes deployed by different operators, and on the coexistence of LTE-U schemes and wireless communication systems such as WiFi. The 3rd Generation Partnership Project (3GPP) requires that fair coexistence of LTE-U schemes and wireless communication systems such as WiFi should be guaranteed, and the spectrum resources in an unlicensed frequency band shall operate as secondary carriers with the assistance of a primary carrier in an licensed frequency band. Listen Before Talk (LBT) has been widely accepted in the industry as a general mean for LTE-U contention access.
In order to provide a flexible, fair and adaptive channel access mechanism, it is required in the European Telecommunications Standards Institute (ETSI) standards that the LBT technology shall operate in the unlicensed frequency bands of 5150-5350 MHz and 5470-5725 MHz. In an LBT process, which is similar to the WiFi Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism, before accessing a channel, each device needs to check whether a signal is currently being transmitted in the channel or not through Clear Channel Assessment (CCA) to determine whether the channel is occupied. Devices accessing spectrum resources in an unlicensed frequency band are categorized by the ETSI standards into frame-based and load-based devices, which correspond to two access mechanisms, respectively: the Frame Based Equipment (FPE) and the Load Based Equipment (LBE).
In the LBE channel access mechanism, extended CCA detection needs to be made before the channel is accessed, that is, a random factor N is generated, and neither will channel be accessed, nor will data transmission be initiated, until the channel has been idle for a period of time which is N times a period of time for CCA. As can be apparent, for continuous transmission each time, both the period of time for which the channel is occupied and the start point where the transmission is started are variable in the LBE channel access mechanism.
As currently required in the 3GPP sessions, LTE transmission in a licensed frequency band, and LTE-U transmission in an unlicensed frequency band need to be aligned in time, while the start point of time when LTE-U signal transmission is started may be at any position in some sub-frame due to factors such as LBT contention access or a period of time for preparing a radio frequency, therefore, a partial sub-frame, i.e., a physical resource with a shorter length of time than the length of a normal sub-frame, may be transmitted during LTE-U transmission. If no signal is transmitted by an LTE-U device in the partial sub-frame during contention access, then the sub-frame would be preempted by another device in the case of drastic contention. Furthermore the longest period of time for which a station transmits continuously is specified as 4 ms in the regulation for unlicensed frequency bands in Japan, and regulations in different regions need to be followed by the 3GPP standard to be set up. Accordingly if the longest period of time for continuous transmission in LTE-U is 4 ms, and no signal is transmitted in two partial sub-frames (i.e., a start sub-frame and an end sub-frame), then the efficiency of LTE-U transmission will be degraded by a factor of 25%, so it is necessary to transmit a signal in a partial sub-frame in an unlicensed frequency band.
However there has been absent so far a solution to transmission of a signal in a partial sub-frame in an unlicensed frequency band.